The present invention relates to an evaporable getter device for cathode-ray tubes (CRTs), used in television sets and monitors.
As known in the art, getter materials are used in any applications wherein the maintenance of vacuum is required for a long time. In particular, CRTs contain evaporable getter materials capable of fixing traces of harmful gases that would compromise proper operation of the CRT.
Trace gases may be left in CRTs during the production stage, even though an evacuation step is performed before final sealing of the tube, or trace gases may come from degassing of the materials forming the tubes.
To remove these trace gases, barium metal is used, which is deposited in the form of a thin film on the internal walls of the CRT. This deposition is accomplished by a so-called evaporable getter device, formed by an open metallic container filled with a mixture of powders of a barium compound, usually BaAl4, and nickel, Ni, capable of releasing barium by evaporation after sealing of the CRT. This mixture is referred to in the following as BaAl4/Ni.
In order to evaporate barium, the container is heated preferably by induction, through a coil placed outside the tube, thus causing an increase in temperature of the powders to about 800° C. At these temperatures a strongly exothermic reaction takes place between BaAl4 and Ni, that causes a further rise of temperature to about 1200° C., at which temperature barium evaporates. The metal then condenses in the form of a film on the conical wall and the mask of the CRT. This barium film is the active element in the gettering of gases.
For optimal working of the CRT it is required that the barium film have a thickness as uniform as possible. A deposit of uneven thickness may have small projections from which, through gas absorption, barium particles may be lost which have a high probability to end up on the electron gun and/or on the mask. In the first case, these particles may cause electric arcs and short circuits, while in the second case, they obstruct the passage of electrons and hence the formation of the image, thus causing the onset of dark spots on the screen. Moreover, a barium film with zones of high thickness has worsened characteristics of saturation by gases, consequently causing a reduction in the absorbing capacity of the getter.
In order to cope with these problems, Italian Patent IT 1,295,896 in the name of SAES Getters S.p.A. describes a baffle that allows diffusion of the barium vapors along the walls of the tube to produce even deposits. Through the use of such a baffle the distribution of barium is improved, that becomes wider, more reproducible and deposited on the walls of the CRT tube without involving the mask and the phosphors-bearing surface. In this case too, however, the barium layer shows a rather uneven thickness, thus not solving some of the above-mentioned drawbacks in a fully satisfactory way.
U.S. Pat. No. 4,128,782 describes a U-shaped device containing a mixture of BaAl4/Ni with which titanium hydride (TiH2) is mixed. When the barium evaporation temperature is reached, TiH2 decomposes and the hydrogen thus formed acts as a diffusing means for the barium atoms that, by repeatedly hitting hydrogen molecules, travel non-linear paths and spread over a wide surface, thus forming deposits with a more regular thickness compared to the devices not containing the hydride. In this case, however, the extra-component, TiH2, subtracts part of the volume available for the BaAl4/Ni mixture. Therefore, with the same dispenser size, there is released inside the CRT a lower barium amount than what would be released without the third component. In addition, titanium hydride is a rather expensive and troublesome material to handle, as it is readily flammable and reacts violently with water. A production process involving such a compound would thus entail problems related to safety and difficult to manageability.